The singlemost uniform diagnostic feature of cystic fibrosis (CF) is an elevation in sweat NaCl concentrations. This does not involve changes in the amount or composition of the primary secretion formed by the sweat gland secretory coil, but results from defective NaCl reabsorption across sweat ducts. Recent studies strongly suggest that impaired Cl permeability underlies this deficiency in NaCl absorption. This proposal will characterize and localize the chloride permeability defect in CF sweat ducts. We will apply a variety of morphologic and electrophysiologic techniques to isolated perfused human sweat ducts in vitro. Specifically, we will characterize the transport-related morphology of normal and CF sweat ducts using differential interference contrast, immunoflourescent and transmission electron microscopy. Transepithelial voltage and resistance will be determined under ion replacement conditions and in response to transport inhibitors to characterize the basic sweat duct transport properties. Transepithelial isotopic fluxes will detect electrically-silent co-transport or exchange processes. Conventional microelectrode techniques will provide a complete equivalent electrical circuit analysis to characterize the conductance properties of the limiting cell membranes. Ion-selective electrode techniques will assay the cellular Cl, Na and K activities and the driving forces for transmembrane transport of these ions. Voltage-induced perturbations of cellular and paracellular morphology will provide independent evidence to localize and characterize conductance pathways. This array of techniques will provide a complete characterization of electrolyte transport mechanisms of normal and CF sweat ducts at the cellular level.